Howdy, and welcome!
When you say "black powder revolver," do you mean a cap-and-ball/percussion revolver that can use loose powder and ball (like a Remington 1858), or do you mean a cartridge revolver with cartridges like .45 Colt or .38 Special loaded with black powder instead of smokeless powder? Either one is a good choice.
Or you can get a percussion gun and a cartridge conversion cylinder, which will let you shoot the same gun either way.
If you are talking about percussion guns, Abilene was correct that percussion caps have been about impossible to find, except by luck, for the past few years. For reloading cartridge guns, primers are available now but pricey, but a couple years ago they were almost unobtainium like percussion caps. But over the past decade or so, percussion caps have sometimes been easier to find than primers, if I remember my history right. (He was also right that you may want to replace the factory nipple for more reliability, regardless of what brand of revolver you buy.)
Either way, percussion or cartridge, calibers that start with "3" will be a little cheaper to shoot than calibers that start with "4." But the bigger calibers give you more "bang" (and flash) for your buck and are more fun for some people, although recoil will be a little more. Cartridges like .44-40 and .45 Colt can be pretty impressive with a full load of black powder. Personally, I wouldn't start with a .31/.32 caliber gun unless maybe it's a .32-20 cartridge revolver, unless you are specifically looking for a very light caliber, simply because the small calibers are very limiting if you're trying to do anything more than poke holes in paper.
There are a lot of different "brands" of revolver out there, but most of the current production ones come from just a few manufacturers. Ruger makes Blackhawk (adjustable sights) and Vaquero (fixed sights) revolvers, and you'll also find Ruger Old Army revolvers on the market, which take powder and ball and can be thought of as a modernized version of (or inspired by) the old Remington revolver. The Ruger Vaquero is likewise a more modern revolver somewhat resembling the Colt Single Action Army (SAA), but it's different enough that it isn't exactly a "replica" although it is more reliable and an excellent gun in its own right, and it certainly still has that cowboy flavor.
Colt revolvers are good but tend to be on the pricier side. USFA guns are also good but pricey, and they are turning into "collector's items" too as they aren't being made anymore.
Most other cowboy-style revolvers (both cap-n-ball and cartridge) in production today are made by either Uberti or Pietta in Italy, even if they have a different brand (like EMF). Some companies, like Taylor's and Cimarron, sell both makes, and sometimes you have to pay attention to know which one you're getting. Pietta and Uberti both make good products these days, although they each have their pros and cons and little quirks.
One thing to know (for percussion or cartridge guns) is that Uberti "open top" style revolvers (meaning revolvers without a top strap, e.g. the 1851 Navy, 1860 Army, and similar guns as well as the open top cartridge revolvers) come from the factory with what's called a "short arbor," which is basically a technical flaw. They require a little work (which some people do themselves if they're savvy) before they function the way they should. Pietta revolver don't tend to have this issue. And it's not relevant to revolvers with a top strap, such as the 1858 Remington "New Army" and the single action army-type cartridge guns.
This only matters to some people, but not every "replica" out there is a replica of something that existed in the 1800s. For example, replica "1851 Navy" revolvers are popular in both .36 and .44 caliber, even though they historically were never made in .44, just .36 caliber. Also, brass-framed percussion revolvers are generally not "authentic" (and are also less durable), and brass trigger guards/backstraps on replicas of the Colt SAA are also not the way they were made "back in the day." But if you're not trying to reenact, that may not matter at all, and some people certainly like the flashy look (and typically lower price). Octagon barrels on replicas of the Colt Single Action Army are also a modern invention, but very popular. (Octagon barrels on percussion revolvers like the 1851 and 1858, on the other hand, are perfectly authentic.)
Hopefully those thoughts are enough to get you started! I don't know if I answered your question, but hopefully you're more informed!
As to brand, it doesn't much matter (although plenty of people will disagree
), except that open top revolvers by Uberti should have the "short arbor" problem fixed. Other than that they're good guns. Pietta's stuff is good quality these days too; they are trying hard not to be "behind" Uberti in quality or fit and finish. Ruger are durable, quality guns inspired by the Old West but not exactly "replicas." They're all good options -- pick one (or more) and have fun!
One other thing -- if you plan to use a revolver a lot, regardless of brand and type, there may be some tweaks/polishing/tuning that can be helpful. This may or may not be something you can do yourself, depending on how much tuning is needed. But if you want to use the gun a lot, or in competetion, it may be wise to have a good gunsmith go over it first.